U.S. patent application number 12/370578 was filed with the patent office on 2009-06-18 for display unit and backlight unit.
This patent application is currently assigned to Sony Corporation. Invention is credited to Norimasa FURUKAWA, Hiroaki Ichikawa, Kenichi Kikuchi.
Application Number | 20090153464 12/370578 |
Document ID | / |
Family ID | 35124316 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090153464 |
Kind Code |
A1 |
FURUKAWA; Norimasa ; et
al. |
June 18, 2009 |
DISPLAY UNIT AND BACKLIGHT UNIT
Abstract
The screen is exempt from variation in brightness due to uneven
temperature distribution. The color liquid crystal display unit has
a color display panel of transmissive type and a backlight unit
placed behind the color display panel. The backlight unit has a
plurality of LEDs connected in series, the drive unit to drive the
LEDs in correspondence with the LEDs, and the temperature sensor to
detect the temperature of LEDs. The groups of LEDs are arranged in
regions where the display unit has the same temperature. The drive
unit controls current to be supplied to LEDs in response to
temperature detected by the temperature sensor so that the LEDs
retain the constant brightness even when the LEDs fluctuate in
temperature.
Inventors: |
FURUKAWA; Norimasa; (Tokyo,
JP) ; Kikuchi; Kenichi; (Kanagawa, JP) ;
Ichikawa; Hiroaki; (Kanagawa, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
Sony Corporation
|
Family ID: |
35124316 |
Appl. No.: |
12/370578 |
Filed: |
February 12, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11176391 |
Jul 8, 2005 |
7511695 |
|
|
12370578 |
|
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Current U.S.
Class: |
345/102 |
Current CPC
Class: |
G09G 2320/064 20130101;
G09G 2320/041 20130101; G09G 2320/0233 20130101; G09G 2320/0633
20130101; G09G 3/342 20130101; G09G 3/3413 20130101; G09G 2360/145
20130101; G02F 1/133603 20130101 |
Class at
Publication: |
345/102 |
International
Class: |
G09G 3/36 20060101
G09G003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2004 |
JP |
JP 2004-205144 |
Claims
1-9. (canceled)
10. A backlight unit installed in the back of a display unit,
comprising: lighting units, each lighting unit comprising: a light
emitting diode, and temperature detecting unit to detect the
temperature of the lighting unit, and a drive unit to control
current supplied to the light emitting diode according to the
temperature of the lighting unit, so that each lighting unit keeps
a uniform brightness even though each lighting unit changes in
temperature, to adjust current to be supplied to the light emitting
diode by means of pulse width modulation, so as to keep at least
the brightness of light emitted from each lighting unit uniform, to
control the peak value of the current to be supplied to the light
emitting diode, so that the resolution of pulse width is the same,
and to adjust the peak value of current to be supplied to the light
emitting diode in response to the detected temperature.
11. The backlight unit as defined in claim 10, wherein the drive
unit controls current supplied to the light emitting diode, so that
each lighting unit keeps a uniform brightness as well as a uniform
chromaticity even though each lighting unit changes in
temperature.
12. The backlight unit as defined in claim 10, wherein the
temperature detecting unit detects the temperature of one of the
lighting units having a malfunctioning temperature sensor according
to information from a memory which previously stored the
temperature of the lighting unit having the malfunctioning
temperature sensor in response to the temperature of one of the
lighting units at the position where a temperature sensor is
installed.
13. The backlight unit as defined in claim 10, further comprising a
light detecting sensor to detect the brightness of light emitted
from the light emitting diode in each lighting unit, wherein the
drive unit adjusts current supplied to the light emitting diodes so
as to keep at least the brightness of light emitted from each
lighting unit uniform.
14. A display unit comprising: a non-luminous screen of
transmissive-type, lighting units, placed behind the screen, each
lighting unit comprising: a light emitting diode, and a temperature
detecting unit to detect the temperature of the lighting unit, and
a drive unit to control current supplied to the light emitting
diode according to the temperature of the lighting unit, so that
each lighting unit keeps a uniform brightness even though each
lighting unit changes in temperature, to adjust current to be
supplied to the light emitting diode by means of pulse width
modulation, so as to keep at least the brightness of light emitted
from each lighting unit uniform, to control the peak value of the
current to be supplied to the light emitting diode, so that the
resolution of pulse width is the same, and to adjust the peak value
of current to be supplied to the light emitting diode in response
to the detected temperature.
15. The display unit as defined in claim 14, wherein the drive unit
controls current supplied to the light emitting diode, so that each
lighting unit keeps a uniform brightness as well as a uniform
chromaticity even though each lighting unit changes in
temperature.
16. The display unit as defined in claim 14, wherein the
temperature detecting unit detects the temperature of one of the
lighting units having a malfunctioning temperature sensor according
to information from a memory which previously stored the
temperature of the lighting unit having the malfunctioning
temperature sensor in response to the temperature of one of the
lighting units at the position where temperature sensor is
installed.
17. The display unit as defined in claim 14, further comprising a
light detecting sensor to detect at least the brightness of light
emitted from the light emitting diode in each lighting unit,
wherein the drive unit adjusts current to be supplied to the light
emitting diode so as to keep at least the brightness of light
emitted from each lighting unit uniform.
18. The display unit as defined in claim 14, which further
comprises an optical system that uniformly mixes the colors of
light emitted from each light emitting diode in each lighting unit,
and wherein the drive unit acquires as a representative value the
brightness or chromaticity detected by the light detecting sensor
and adjusts current to be supplied to each light emitting diode in
each lighting unit so that the lighting unit changes uniformly as a
whole and the light emitted from each lighting unit remains
constant in brightness or chromaticity.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a display unit with a
non-luminous screen of transmissive-type and also to a backlight
unit placed behind the screen.
[0002] Liquid crystal panels are usually provided with a backlight
unit composed of cold cathode fluorescent lamps (CCFL). However,
there is a demand for a mercury-free backlight unit from the
standpoint of environmental protection. Light emitting diodes
(LEDs) are regarded as promising in this respect as a substitute
for CCFLs. Application of LEDs to televisions is under meticulous
research because of their ability to produce a well-balanced white
color by optical color mixing from LEDs emitting red, green, and
blue elementary colors.
[0003] The disadvantage of using LEDs as the light source is the
necessity of supplying red LEDs, green LEDs, and blue LEDs with
current independently because they differ in luminous efficiency.
Moreover, LEDs vary in their semiconductor compositions depending
on their emitting color and hence vary in their driving voltage and
power consumption. However, it is not practical to individually
drive LEDs constituting the backlight unit.
[0004] In practical use of LEDs as the light source of backlight
unit, it is common practice to divide LEDs into a certain number of
groups and drive each group (in which LEDs are connected in series)
as a whole.
[0005] In other words, each group consists of a prescribed number
of red, green, and blue LEDs connected in series, and each group is
connected to a DC-DC converter power source and a PWM control unit.
Thus it is possible to adjust the tint and brightness of backlight
(which result from combination of red, green, and blue) by
adjusting the current (through PWM control) supplied to the LEDs in
each group.
[0006] Patent Document 1:
[0007] Japanese Patent Laid-open No. 2001-272938
SUMMARY OF THE INVENTION
[0008] Incidentally, LEDs vary in amount of light emission
depending on temperature and this temperature dependence also
varies depending on the color they produce. FIG. 15 is a graph
showing the fundamental temperature characteristics of red, green,
and blue LEDs. In FIG. 15, X-axis represents the element
temperature and Y-axis represents the relative brightness (or
output). The relative brightness is expressed in terms of percent,
with 100% denoting the output of the element at 25.degree. C.
[0009] The red LED is a semiconductor in layer structure which is
composed of four elements (Al, In, Ga, and P). Having a low band
gap energy, it decreases in the amount of light emission at a high
temperature owing to decrease in the amount of carriers
contributing to light emission. Consequently, at about 75.degree.
C. for ordinary light emission its luminance decreases to about 70%
of that at normal temperature (25.degree. C.). This change is much
larger than change in green and blue LEDs.
[0010] By contrast, green and blue LEDs composed of three elements
(In, Ga, and N) are less vulnerable to temperature change because
of their large band gap energy resulting from their shorter
wavelengths than red and closer to violet. In fact, it is noticed
from FIG. 15 that the temperature characteristics of the blue
LED(B) is almost flat.
[0011] The LEDs as the light source for backlight of liquid crystal
display units are usually run in such a way that individual LEDs
are supplied with a large amount of electric power so as to reduce
the total number of LEDs. In addition, the LEDs are connected in
series and hence each series greatly fluctuates in load due to heat
resistance. As the result, the LEDs fluctuate in heat generation
and hence in temperature moment by moment. Consequently, the red
LEDs greatly decrease in luminance, whereas the blue LEDs almost
remain unchanged in luminance.
[0012] Also, the surrounding air in the display unit is warmed up
by the heat generated by LEDs and the relative density of the air
decreases. As a result, the air in the display unit rises
upwards.
[0013] Accordingly, in case which LEDs are used as the light source
for backlight of the liquid crystal display units, the liquid
crystal display unit with a backlight of LEDs gets hot in the upper
part and remains cool in the lower part. The hot upper part
decreases in red light output and hence becomes bluish, and the
cool lower part remains the same in red light output. As the
result, the display unit varies in brightness and tint from its
upper part to its lower part.
[0014] This problem becomes serious in proportion to the display
size.
[0015] One way to tackle this problem is to eliminate the variation
of display brightness due to temperature distribution.
[0016] The gist of the present invention resides in a backlight
unit installed in the back of a display unit, which includes a
lighting unit composed of a plurality of LEDs connected in series,
a drive unit to drive LEDs in the lighting unit, each drive unit
corresponding to each lighting unit, and a temperature detecting
unit to detect the temperature of each lighting unit, the lighting
units being arranged in various positions at which the LEDs keep a
prescribed temperature, and the drive units controlling current to
be supplied to the LEDs in each lighting unit according to the
temperature of each lighting unit, which is detected by the
temperature detecting unit, so that each lighting unit keeps
brightness uniform even though each lighting unit changes in
temperature.
[0017] According to one embodiment of the present invention, the
LEDs in each lighting unit are arranged in series horizontally and
the lighting units are arranged at positions where the temperature
is approximately the same because of the temperature distribution
resulting from the heat generation of the lighting unit per se.
Therefore, the drive unit controls current to be supplied to LEDs
in each lighting unit according to the temperature, which is
detected by the temperature detecting unit, thereby keeping the
backlight unit uniform in brightness even though the lighting unit
changes in temperature.
[0018] The gist of the present invention resides also in a display
unit which includes a non-luminous screen of transmissive-type, a
lighting unit which is placed behind the screen and is composed of
a plurality of LEDs connected in series, a drive unit to drive LEDs
in each lighting unit, the drive unit corresponding to each
lighting unit, and a temperature detecting unit to detect the
temperature of each lighting unit, the lighting units being
arranged in various positions at which the LEDs keep a prescribed
temperature, and the drive units controlling current to be supplied
to the LEDs in each lighting unit according to the temperature of
each lighting unit, which is detected by the temperature detecting
unit, so that each lighting unit keeps at least brightness uniform
even though each lighting unit changes in temperature.
[0019] The backlight unit and display unit according to the present
invention employ, as the light source, lighting units each composed
of a plurality of LEDs connected in series. The LEDs constituting
each lighting unit are arranged in varied positions where they keep
a prescribed temperature, and the drive units control current to be
supplied to the LEDs in each lighting unit, so that each lighting
unit keeps a uniform brightness even though each lighting unit
changes in temperature.
[0020] The backlight unit and display unit constructed as mentioned
above retain a constant brightness and tint irrespective of
temperature distribution on the screen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic perspective view showing the structure
of the color liquid crystal display unit of backlight type to which
the present invention has been applied.
[0022] FIG. 2 is a block diagram showing the structure of the drive
circuit of the color liquid crystal display unit mentioned
above.
[0023] FIG. 3 is a schematic diagram showing the arrangement of
LEDs in the backlight unit constituting the color liquid crystal
display unit mentioned above.
[0024] FIG. 4 is a schematic diagram showing the arrangement of
LEDs mentioned above in which individual LEDs are represented by
diode symbols for circuit diagrams.
[0025] FIG. 5 is a schematic diagram showing a unit cell in which
six diodes (two each of red, green, and blue diodes) are arranged
in one line.
[0026] FIG. 6 is a schematic diagram showing three unit cells 4
joined together.
[0027] FIG. 7 is a schematic diagram showing the actual connection
of LEDs in the light source 21 of the backlight unit.
[0028] FIG. 8 is a schematic diagram showing the connection of LEDs
in the backlight unit mentioned above.
[0029] FIG. 9 is a schematic diagram showing the temperature
distribution in the display unit.
[0030] FIG. 10 is a schematic diagram in which the temperature
distribution in the display unit is superposed on the connection of
LEDs in the backlight unit.
[0031] FIG. 11 is a diagram which is used to estimate the
temperature at each position from one temperature sensor and the
temperature distribution pattern.
[0032] FIG. 12 is a diagram showing the drive circuit to drive
LEDs.
[0033] FIGS. 13A to 13C are diagrams showing PWM pulses to drive
individual red, green, and blue LEDs.
[0034] FIGS. 14A to 14C are diagrams showing signals to drive
individual red, green, and blue LEDs, with peak values and PWM
controlled.
[0035] FIG. 15 is a diagram showing the fundamental temperature
characteristics of red, green, and blue LEDs.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The embodiments of the present invention will be described
in more detail with reference to the accompanying drawings.
[0037] The present invention is applicable to the color liquid
crystal unit 100 of backlight type constructed as shown in FIG.
1.
[0038] The color liquid crystal display unit 100 consists of a
color display panel 10 of transmissive type and a backlight unit 20
placed behind the color display panel 10.
[0039] The color liquid crystal display panel 10 of transmissive
type has the TFT substrate 11 and the opposed electrode substrate
12, which face each other with the liquid crystal layer 13 (of
twisted nematic TN liquid crystal) interposed between them. The TFT
substrate 11 has signal lines 14 and scanning lines 15 formed
thereon in a matrix pattern. At their intersections are formed
thin-film transistors 16 (as switching elements) and pixel
electrodes 17. The thin film transistors 16 are sequentially
selected by the scanning lines 15; they also write the image
signals (supplied from the signal lines 14) into the corresponding
pixel electrodes 17. On the inside of the opposed electrode
substrate 12 are formed opposed electrodes 18 and color filters
19.
[0040] The color liquid crystal display unit 100 has the color
liquid crystal display panel 10 of transmissive type mentioned
above, which is held between two polarizers and illuminated with
white light from its rear by the backlight unit 20. Upon active
matrix driving, it produces full-color images as desired.
[0041] The backlight unit 20 has the light source 21 and the
wavelength selecting filter 22, so that it illuminates (from rear
through the wavelength selecting filter 22) the color liquid
crystal panel 10 with the light emitted from the light source
21.
[0042] The color liquid crystal display unit 100 is driven by the
drive circuit 200 whose electrical block diagram is shown in FIG.
2.
[0043] The drive circuit 200 consists of the power source 110 (to
supply the color liquid crystal display panel 10 and the backlight
unit 20 with electric power), the X driver circuit 120 and the Y
driver circuit 130 (to drive the color liquid crystal display panel
10), the RGB processing unit 150 (to be supplied with external
image signals through the input terminal 140), the image memory 160
and the controller 150 (which are connected to the RGB processing
unit 150), and the backlight control unit 180 (to drive and control
the backlight unit 20).
[0044] In the drive circuit 200, the image signals entered through
the input terminal 140 are processed such as a chromatic process or
the like by the RGB processing unit 150 and the processed signals
are converted from composite signals into RGB separate signals
suitable for the driving of the color liquid crystal display panel
10. The RGB separate signals are supplied to the control unit 170
and also to the X driver 120 through the image memory 160. The
control unit 170 controls the X driver 120 and Y driver 130 at a
prescribed timing corresponding to the RGB separate signals. The
RGB separate signals supplied to the X driver 120 through the image
memory 160 drive the color liquid crystal display panel 10, thereby
producing images corresponding to the RGB separate signals.
[0045] The backlight unit 20 is placed behind the color liquid
crystal display panel of transmissive type 10, so that it
illuminates directly the rear of the color liquid crystal display
panel 10. The light source 21 of the backlight unit 20 consists of
a plurality of light emitting diodes (LEDs). These LEDs are divided
into several groups, which are driven individually.
[0046] The LEDs constituting the light source 21 of the backlight
unit 20 are arranged in the following manner.
[0047] FIG. 3 schematically shows the arrangement of LEDs. Each
unit cell (4-1 and 4-2) has six LEDs (two each of red LEDs 1, green
LEDs 2, and blue LEDs 3) which are arranged in one line.
[0048] In this case mentioned above, the unit 4 has six LEDs.
However, the number of LEDs for each color in one unit cell is not
limited to the one mentioned above; it may be properly varied to
produce a well-balanced white color (mixed color) according to the
rating and emitting efficiency of LEDs employed.
[0049] In the case shown in FIG. 3, the unit cells 4-1 and 4-2 have
the identical arrangement of LEDs, and they are connected to each
other at the center (indicated by arrows). FIG. 4 shows the unit
cells 4-1 and 4-2 connected to each other, in which LEDs are
represented by symbols for electric circuit drawings. The red LEDs
1, green LEDs 2, and blue LEDs 3 are connected in series
separately, with their polarity conforming to the direction of
current flow (left to right).
[0050] FIG. 5 shows one unit cell 4 consisting of six LEDs (two
each of red LEDs 1, green LEDs 2, and blue LEDs 3) which are
arranged in one line. Each set of two LEDs is represented by 2G,
2R, and 2B as shown in FIG. 5, and the set (2G 2R 2B) denotes the
basic unit as the pattern of six LEDs including two each of green,
red, and blue. In FIG. 6, three unit cells 4 joined in series is
represented by 3*(2G 2R 2B) or (6G 6R 6B).
[0051] The LEDs constituting the light source 21 of the backlight
unit 20 are connected in the following manner.
[0052] The LEDs in the light source 21 are grouped into
intermediate units (6G 6R 6B) which are arranged in a matrix
pattern (five horizontal rows and four vertical columns), as shown
in FIG. 7. Each intermediate unit consists of three basic units (2G
2R 2B). Therefore, there are 360 LEDs in total.
[0053] These intermediate units (6G 6R 6B) are electrically
connected in the horizontal direction on the screen. The
arrangement of intermediate units in this manner results in a
plurality of the groups of LEDs 30 connected side by side
horizontally formed in the light source 21 of the backlight unit 20
as shown in FIG. 8.
[0054] Each group of LEDs 30 connected in series horizontally is
provided with the independent LED drive circuit 31 in the backlight
unit 20. The LED drive circuit 31 supplies current to the groups of
LEDs 30, thereby causing them to emit light.
[0055] The groups of LEDs 30 horizontally connected in series are
arranged such that those LEDs in a certain region have
approximately the same temperature when the temperature
distribution is gauged in the backlight unit 20.
[0056] FIG. 9 shows the temperature distribution on the screen of
the color liquid crystal display unit 100, with the backlight unit
20 working. Densely hatched parts indicate the region of high
temperature and lightly hatched parts indicate the region of low
temperature. It is noted from FIG. 9 that the color liquid crystal
display unit 100 has a high temperature in its upper region and a
low temperature in its lower region.
[0057] FIG. 10 is a schematic diagram in which the temperature
distribution in the display unit (which is shown in FIG. 9) is
superposed on the connection of LEDs in the backlight unit (which
is shown in FIG. 8). It is noticed from FIG. 10 that connecting
LEDs in the horizontal direction is equivalent to connecting LEDs
at approximately the same temperature.
[0058] As shown in FIG. 10, the backlight unit 20 is provided with
temperature sensors 32 to detect the temperature of each group of
LEDs 30. There are a plurality of temperature sensors 32 arranged
vertically, each corresponding to the group of LEDs 30 horizontally
connected in series. The arrangement of temperature sensors 32 may
be modified as shown in FIG. 11. The display unit shown in FIG. 11
has only one temperature sensor 32 at its center, and it also has a
memory recording the pattern of temperature distribution in the
vertical direction. The temperature sensor 32 can estimate from the
detected value of the temperature at different regions across the
screen in the vertical direction, by referencing the content of the
memory. The information about temperature detected by the
temperature sensor 32 is sent to the LED drive circuit 31 that
drives the corresponding groups of LEDs 30.
[0059] In addition, as shown in FIG. 10, the backlight unit 20 is
provided the light quantity sensor or chromaticity sensor 33, which
detects the luminance or chromaticity of each group of LEDs 30. The
embodiment shown in FIG. 10 has a plurality of luminance or
chromaticity sensors 33, each corresponding to the groups of LEDs
30 horizontally connected in series. This embodiment may be
modified such that the backlight unit 20 has only one luminance or
chromaticity sensor 33 if it is provided with a diffuser panel that
uniformly and efficiently mixes the colors of individual LEDs. In
this case, the luminance or chromaticity value detected by the
luminance or chromaticity sensor 33 is supplied to the LED drive
circuit 31 that drives the corresponding groups of LEDs 30.
However, it is possible to use only one luminance or chromaticity
sensor 33 if the backlight unit 20 is provided with a diffuser
panel or any other optical system that uniformly and effectively
mixes LEDs colors.
[0060] The LED drive circuit 31, which is placed in the backlight
drive control unit 180, drives the groups of LEDs 30 which are
horizontally connected in series in the following manner.
[0061] FIG. 12 shows an example of the LED drive circuit 31.
[0062] The LED drive circuit 31 has the DC-DC converter 41, the
constant resistance (R.sub.c) 42, the FET 43, the PWM control
circuit 44, the capacitor 45, the sample holding FET 46, the
resistance 47, and the hold timing circuit 48.
[0063] The DC-DC converter 41 receives a DC voltage V.sub.IN from
the power source 110 shown in FIG. 2. Then it converts its input
(DC power) into a stabilized DC output voltage V.sub.cc by
switching. In other words, the DC-DC converter 41 generates a
stabilized output voltage V.sub.cc in such a way that a
predetermined value (V.sub.ref) of potential difference is obtained
between the voltage entered through the feedback terminal V.sub.f
and the output voltage V.sub.cc.
[0064] The anode of the group of LEDs 30 connected in series is
connected to the output terminal for output voltage V.sub.cc of the
DC-DC converter 41 through the constant resistance (R.sub.c). In
addition, the anode of the group of LEDs 30 connected in series is
also connected to the feedback terminal of the DC-DC converter 41
through the source-drain of the sample holding FET 46. The cathode
of the group of LEDs 30 connected in series is grounded through the
source-drain of the FET 43.
[0065] The gate of the FET 43 receives PWM signals from the PWM
control circuit 44. The FET 43 works in such a way that the
source-drain channel becomes on when the PWM signal is on, and the
source-drain channel becomes off when the PWM signal is off.
Therefore, the FET 43 supplies current to the groups of LEDs 30
when the PWM signal is on and shuts off current to the groups of
LEDs 30 when PWM signal is off. In other words, the FET 43 causes
the groups of LEDs 30 to emit light when the PWM signal is on and
causes the groups of LEDs 30 to suspend light emission when the PWM
signal is off.
[0066] The PWM control circuit 44 generates the PWM signal as a
binary signal to control the duty ratio of on-time and off-time.
The PWM control circuit 44 receives the value of light quantity
detected by the luminance sensor 33, and it controls the pulse
width of PWM signal so that the luminance of the groups of LEDs 30
connected in series coincides with the predetermined value of
luminance. In other words, the PWM control circuit 44 increases the
pulse width, thereby increasing the duration of light emission of
the groups of LEDs 30, when the value of light quantity detected by
the luminance sensor 33 is lower than the predetermined value of
luminance. It also reduces the pulse width, thereby reducing the
duration of light emission of the groups of LEDs 30, when the value
of light quantity detected by the luminance sensor 33 is higher
than the predetermined value of luminance.
[0067] The capacitor 45 is placed between the output terminal and
the feedback terminal of the DC-DC converter 41. The resistance 47
is connected to the output terminal of the DC-DC converter 41 and
the gate of the sample holding FET 46.
[0068] The hold timing circuit 48 receives the PWM signal and
generates the hold signal which is off for a prescribed period of
time at the edge of rise of the PWM signal and which is on
otherwise.
[0069] The gate of the sample holding FET 46 receives the hold
signal delivered from the hold timing circuit 48. The sample
holding FET 46 works in such a way that the source-drain becomes on
when the hold signal is off and the source-drain becomes off when
the hold signal is on.
[0070] The LED drive circuit 31 mentioned above supplies current
I.sub.LED to the groups of LEDs 30 only when the PWM signal from
the PWM control circuit 44 is on. The sample holding circuit is
composed of the capacitor 45, the sample holding FET 46, and the
resistance 47. The sample holding circuit samples the value of
voltage at the anode of the groups of LEDs 30 (that is, at one end
of the resistance 42 which is not connected to the output voltage
V.sub.cc) when the PWM signal is on and then supplies it to the
feedback terminal of the DC-DC converter 41. The DC-DC converter 41
is designed to stabilize the output voltage V.sub.cc according to
the voltage value entered to the feedback terminal. Therefore, it
is possible to make the peak value constant for the current
I.sub.LED that flows to the constant resistance R.sub.c 42 and the
groups of LEDs 30.
[0071] Consequently, the LED drive circuit 31 performs pulse
driving according to the PWM signals while keeping the peak value
constant for the current I.sub.LED flowing into the groups of LEDs
30. Thus, the LED drive circuit 31 is so controlled as to make the
luminance of light emitted from the groups of LEDs 30 connected in
series equal to the predetermined value of luminance according to
the amount of light detected by the luminance sensor 33.
[0072] Moreover, the LED drive circuit 31 also has the desired
value control circuit 49, which is designed to adjust the
stabilized voltage value V.sub.ref for the output voltage V.sub.cc
of the DC-DC converter 41 according to the value of temperature
output from the temperature sensor 32.
[0073] The desired value control circuit 49 receives the value of
temperature of the groups of LEDs 30 connected in series which has
been detected by the temperature sensor 32. The desired value
control circuit 49 has the table 50 which stores values indicating
the change in luminance for change in temperature. While
referencing this table 50, it generates the desired voltage value
V.sub.ref so that the groups of LEDs 30 keep a constant luminance
even though they change in temperature. The desired voltage value
V.sub.ref generated by the desired value control circuit 49 is
given to the DC-DC converter 41.
[0074] The DC-DC converter 41 generates the output voltage V.sub.cc
so that the desired voltage V.sub.ref is obtained for the potential
difference between the voltage entered from the feedback terminal
V.sub.f and the output voltage V.sub.cc.
[0075] The LED drive circuit 31, which is provided with the desired
value control circuit 49 as mentioned above, adjusts the peak value
of the current I.sub.LED flowing to the groups of LEDs 30 in
response to the desired voltage value V.sub.ref generated by the
desired value control circuit 49. Thus, the groups of LEDs 30
connected in series emit light at a constant luminance, without
change in pulse width of PWM signals, even though they change in
temperature.
[0076] According to the embodiment mentioned above, the color
liquid crystal display unit 100 has the groups of LEDs connected in
series under control by the same drive circuit, which are arranged
in the region of the same temperature immediately behind the
display unit. In addition, the color liquid crystal display unit
100 adjusts current for the groups of LEDs so as to retain a
constant luminance even though the temperature changes. To be
concrete, it performs PWM control for drive current so that the
LEDs emit light with a constant luminance and it also adjusts the
peak value of drive current so that the LEDs emit light with a
constant quantity of light even though they change in
temperature.
[0077] Thus, the color liquid crystal display unit 100 is intended
to eliminate the fluctuation in luminance on the screen which
results from variation in temperature.
[0078] The LED drive circuit 31 may be modified as follows.
[0079] As mentioned above, the backlight unit 20 is designed such
that a certain number of LEDs connected in series are driven as a
whole.
[0080] LEDs for red (R), green (G), and blue (B) primary colors
need different drive circuits because they differ in emission
efficiency. In other words, they differ in power consumption and
emission efficiency. Therefore, the pulse width adjusted by PWM
control varies for different colors. For example, about 50% of
on-time is enough red LEDs immediately after lighting because red
LEDs have high emission efficiency at a low temperature, whereas
about 80-90% of on-time is necessary for blue LEDs which are poor
in emission efficiency. In other words, different drive circuits
are necessary for different colors.
[0081] Since the pulse width adjusted by PWM control for each color
varies, it follows that the resolution assigned to adjustment of
the pulse width for PWM control differs for the luminance of the
groups of LEDs 30. For example, the resolution is rough for red and
fine for blue. This means that the accuracy of adjustment varies
depending on colors. The uneven accuracy of resolution for
different colors is a hindrance to uniform white color.
[0082] FIGS. 13A to 13C show the difference in resolution in PWM
control for different colors.
[0083] In FIG. 13A to 13C, it is assumed that the PWM control pulse
can be divided in 256 steps (8 bits) to adjust a pulse width. FIG.
13A shows the PWM control pulse for red; FIG. 13B shows the PWM
control pulse for green; and FIG. 13C shows the PWM control pulse
for blue. It is assumed that it is necessary to mix the red, green,
and blue colors in the following ratio to obtain the desired white
color.
Blue with a PWM width of 256 (100% duty) Green with a PWM width of
191 (75% duty) Red with a PWM width of 126 (50% duty) The blue
color can be adjusted in 256 steps, whereas the red color can be
adjusted only in 126 steps (or 7 bits). In addition, one-step width
for blue corresponds to two-step width for red color. It is
possible to improve the resolution by increasing the number of
bits; however, this approach is not practical because it needs an
expensive converter with a high accuracy.
[0084] For the reasons mentioned above, the LED drive circuit 31
changes the peak value of current (supplied to the groups of LEDs
30) so that the resolution of pulse width is the same for red,
green, and blue LEDs. This is accomplished in practice by entering
the control value of PWM into the desired value control circuit 49
to adjust the desired voltage value V.sub.ref at which the output
voltage V.sub.cc remains stable.
[0085] If it is assumed that when the mixing ratio of red, green,
and blue colors is adjusted for the same peak value of current, the
PWM width for blue is 256 (100% duty), the PWM width for green is
191 (75% duty), and the PWM width for red is 126 (50% duty), then
the peak value of current for red color should be adjusted to 50%
as shown in FIG. 14A, the peak value of current for green color
should be adjusted to 75% as shown in FIG. 14B, and the peak value
of current for blue color should be adjusted to 75% as shown in
FIG. 14C.
[0086] By changing the peak value of current for the groups of LEDs
30, it is possible to have the same resolution for red, blue, and
green at the time of adjustment and hence it is possible to keep a
balance for the accuracy of control on different colors.
* * * * *